The formation of myotendinous junctions (MTJs) is a fundamental component of a functional musculoskeletal system. Muscles interact with tendons at their attachment sites, causing a reorganization of the extracellular matrix (ECM) into connective tissue structures that can bear the large forces exerted by muscle contraction. The transcription factor Scleraxis (Scx) specifies early tendon progenitors and regulates ECM production, and we recently discovered an ECM protein called Thrombospondin-4b (Thbs4b) in zebrafish that is regulated by Scx and required for muscle attachment. How tendon progenitors reach sites of muscle attachment and influence ECM assembly and muscle adhesion at the MTJ remains unclear. The current proposal addresses these issues using the advantages of the zebrafish for in vivo imaging and genetic manipulation. The long-term goal of the proposed research is to understand the spatial dynamics of ECM assembly and Integrin (Itg) signaling at MTJs. Two primary hypotheses guide the research: 1) Scx controls tenocyte specification and ECM production that reorganize Itg signaling components at MTJs, and 2) Thbs4 is a critical Itg ligand in the ECM that acts as a scaffold for other matrix proteins during the formation of MTJs. Aim 1 is to visualize tendon precursors in living zebrafish embryos and study roles for Scx in ECM production and reorganization at MTJs. The optical clarity of zebrafish makes them uniquely suited for this study. Aim 2 analyzes Thbs4b functions in MTJ formation, by altering its expression and analyzing the spatial dynamics of ECM proteins and Itg signaling. Each aim combines novel genetic manipulation, fluorescence dynamics imaging and quantitative methods for physiological stimulation of muscles to get at the mechanisms of ECM assembly at the MTJ.